This invention relates to thin-film deposition, which is widely practical in manufacturing products such as magnetic recording disks. Magnetic recording disks such as hard-disks and flexible disks are widely used as storage media in computers. Magnetic recording disks have basic structures where magnetic recording layers are provided on disk-shaped substrates.
Manufacture of a magnetic recording disk will be described as follows, taking a hard-disk as an example. Conventionally, a substrate made of aluminum is employed in manufacturing a hard-disk. A NiP (nickel phosphide) film is deposited on the substrate. On the NiP film, such an underlying film as a CoCr film is deposited. On the underlying film, such a magnetic film as a CoCrTa film is deposited for the magnetic recording layer. On the magnetic film, a carbon film having a structure similar to diamond, which is called “diamond-like-carbon (DLC) film”, is deposited as a protection layer called “overcoat”.
In manufacture of magnetic recording disks, several limitations are foreseen from the point of view of increasing recording density. Recent recording density in magnetic recording disks has been soaring remarkably. Currently it is reaching 35 gigabit/inch2, supposedly 100 gigabit/inch2 in the future. For higher recording density, it is necessary to make magnetic domains shorter and track width narrower in the longitudinal recording that is generally adopted. For making magnetic domains shorter and track width narrower, it is required to reduce distance between a magnetic head, which is for write-and-readout of information, and a magnetic recording layer. This distance is often called “spacing” in this field. The length of each magnetic domain is often called “bit length”. If spacing is wider at shorter bit length and narrower track width, write-and-readout errors may take place because magnetic flux cannot be captured sufficiently by the magnetic head.
Factor of magnetization-transition region is also important in increasing recording density. In the longitudinal recording, magnetic domains are magnetized alternatively to opposite directions along a track. Each boundary of the magnetic domains does not demonstrate clear linearity within width of the track. This is because the magnetic film is collectively made of fine crystal grains. Each boundary is formed with outlines of crystal grains. Therefore, each boundary is zigzag-shaped. Each boundary of magnetic domains is called “magnetization-transition region” because it is the place where magnetization is inverted. Because each boundary is zigzag-shaped, magnetization transition averaged in track width tends to be not steep but gentle. This means magnetization-transition region is wide. When magnetization-transition region becomes wider, the number of the magnetic domains capable of being provided in limited length of a track becomes smaller. Therefore, the factor of magnetization-transition region lies as a bottleneck in enhancing recording density.
To narrow magnetization transition region, it is required to deposit a magnetic film of smaller crystal grains. For making grains smaller, to make a magnetic film thinner is one solution. However, when grains are made smaller, the problem of thermal decay of magnetization becomes more serious. This point will be described as follows. When a magnetic domain is magnetized, theoretically the magnetization is sustained unless the inverse magnetic field is applied to it. Practically, however, the magnetization is dissolved slightly and slightly from the thermal decay as time passes. Therefore, permanent sustenance of the magnetization is impossible unless the magnetic domains are cooled at the absolute zero temperature. If the problem of the thermal decay appears extremely, recorded information may vanish partially after several years have passed. Such the result is greatly serious in case that the magnetic recording disk is used for semi-permanent information storage.
The thermal decay is the phenomenon of the thermal magnetic relaxation that magnetized particles are magnetized inversely from their thermal oscillation. Particularly, magnetized particles adjacent to a magnetization-transition region have high possibility of the thermal relaxation, i.e. the inverse magnetization, from influence of the inverse field by a neighboring magnetic domain. In magnetic films for magnetic recording, such the thermal decay may take place easily when the grains are made smaller, because each grain becomes thermally unstable. Therefore, unless the problem of the thermal decay is solved, to make magnetization-transition steeper by making grains smaller may suffer difficulty.